Hadith 3:204

Narrated Mutarrif from 'Imran Ibn Husain:

That the Prophet asked him (Imran) or asked a man and Imran was
listening, "O Abu so-and-so! Have you fasted the last days of this
month?" (The narrator thought that he said, "the month of Ramadan").
The man replied, "No, O Allah's Apostle!" The Prophet said to him,
"When you finish your fasting (of Ramadan) fast two days (in
Shawwal)." Through another series of narrators 'Imran said, "The
Prophet said, '(Have you fasted) the last days of Sha'ban?"